In conventional programmable monitors that are associated with PCs, programs are developed by referring to standard symbols which form the fundamental elements of the Sequential Function Chart (SFC). Examples of such elements as shown in FIG. 4 include steps 10, transitions 12, converges 14 or deconverges to name a few. These elements are used as building blocks to represent a program on a display. In addition to employing these elements, individual numbers, as shown in the flow chart 20 illustrated in FIG. 5, can be employed as well. However, only one fundamental element can be displayed in each display area 110a or 110b (area a) which lines up vertically on the flow chart 20.
As a result, present programmable monitors have several disadvantages. First, in order to use the SFC format the user is required to make a large number of key operations to move through the particular SFC transactions identified on the display. Second, the resulting display area is inefficiently used and contains many blank spaces. Third, significant differences in the display size results. For a program having many converges a large amount of space is taken up in the display and therefore the display means becomes even less efficient than for a program having few converges. Thus, complex programs exhibiting a large number of converges become burdensome for the user to develop and cumbersome for the user to review.
A further problem with present programmable monitors is that when a user wishes to debug his program, the system will often limit the user to a display of the functional status of each of the contacts and/or channels effected by the program that are designated by the user. An example of this deficiency is shown in FIG. 11, which represents a typical display 600 which shows the status of a plurality of contacts 822. The status of each of the contacts 822 is represented either by an on-state 826 or an off-state 828 condition in a time chart 825. Sampling times represented on line 832 can be only conventional sampling times such as microseconds, milliseconds, seconds or alternatively cycles. In addition, the display also shows the status of each channel 824. The status is typically represented in hexadecimal format 830 coordinated by the designated time periods 832.
The disadvantage of the display of FIG. 11 is that a conventional monitoring unit can only display the status of the contacts or channels individually. The status of transactions represented on a respective SFC diagram, however, are not shown. To determine the SFC status, therefore, the user must analyze the channel and contact data and correlate that data with the SFC transaction. Such an effort becomes extremely burdensome for the programmer. Moreover, it may ultimately limit that programmer's understanding of the program only to its microscopic effects rather than provide that user with an understanding of the program's effects at the transaction level. As a result, it becomes difficult for the user to debug a program efficiently, thereby eliminating the advantages of structural programming techniques, such as SFC.